In recent years, a technique for measuring a sample to be measured in a nondestructive manner using electromagnetic waves in the frequency band from 0.03 THz or higher to 30 THz or lower (what is called terahertz electromagnetic waves, hereinafter also simply referred to as terahertz waves) has been developed. Specifically, the electromagnetic waves in this frequency band have been applied to, for example, an imaging technique for safe see-through examination that can substitute for fluoroscopy that affects human bodies, a technique that allows evaluation of the bonding state, the density, the mobility, etc. of a carrier in a substance by determining an absorption spectrum and a complex dielectric constant in the substance, and measurement of the membrane thickness of a sample.
Eric R Mueller et al. “2.5 THZ Laser Local Oscillator for the EOS CHEM 1 Satellite”, Proc. of the Ninth International Symposium on Space Terahertz Technology, p. 563 (1998) (hereinafter referred to as NPL 1) discloses a terahertz-wave generating apparatus in the form shown in FIG. 12.
The terahertz-wave generating apparatus shown in FIG. 12 irradiates a gas gain medium 1201, such as methanol, with exciting light 1204 using a CO2 laser unit 1205 as a photo-exciting source to generate terahertz waves. The terahertz waves are amplified between a pair of mirrors 1202 and 1203 that constitute a resonator provided outside a gas cell 1207 in which the gain medium 1201 is sealed to output terahertz waves (laser waves) 1206 from the resonator due to induced emission. The gas cell 1207 is formed of a tube having a relatively large diameter, in which the electromagnetic waves seem to propagate freely.
On the other hand, Japanese Patent Laid-Open No. 2007-88384 (hereinafter referred to as PTL 1) discloses a laser beam source that emits a laser beam to a hollow fiber that contains rare gas to photoelectrically ionize the rare gas, thereby extracting ultraviolet light from an optical resonator.
The terahertz-wave generating apparatus disclosed in NPL 1 uses a cell with a length of 1.5 m as a cell that contains gas serving as a gain medium, and hence it is a large-sized apparatus whose occupation area is of the order of meter, thus being expensive. Furthermore, since the gas has a large capacity, and the mechanism for enclosing light is only a pair of mirrors placed in the laser oscillating direction, the power conversion efficiency for generating terahertz laser waves remains at about 0.7%, which is insufficient in reality.
Since the technique disclosed in PTL 1 is designed to irradiate the rare gas contained in the limited region in the hollow fiber with a laser beam to generate ultraviolet light, a compact apparatus configuration is possible as compared with a method of exciting gas in a free space. However, PTL 1 discloses an apparatus intended to generate ultraviolet light and has no intention to generate terahertz waves.